Question: What political border does the Congress of Vienna have cause to be interested in?
Paraphrase: Which is the political border Congress of Vienna have cause?
SPARQL (Wikidata):  select distinct ?sbj where { ?sbj wdt:P828 wd:Q46362 . ?sbj wdt:P31 wd:Q1292279 } 
Answer: Border Kingdom of Saxony-Kingdom of Prussia 1815